Add simple dropdown component and change severity filter to use it

What does this MR do and why?

This MR adds simple_dropdown.vue and updates severity_filter.vue to use it. There are a bunch of vulnerability report filters that repeat the same boilerplate code, and simple_dropdown.vue refactors them into a reusable component.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #383346 (closed)

ee/spec/frontend/security_dashboard/components/shared/filters/severity_filter_spec.js

@@ -26,13 +26,13 @@ describe('Severity Filter component', () => {
         querystringKey: 'severity',
       });
     });
-  });
 
-  it('emits the filter-changed event when the dropdown updates selected IDs', async () => {
-    const ids = ['A', 'B', 'C'];
-    findDropdown().vm.$emit('selected-changed', ids);
-    await nextTick();
+    it('emits the filter-changed event when the dropdown changes selected IDs', async () => {
+      const ids = ['A', 'B', 'C'];
+      findDropdown().vm.$emit('selected-changed', ids);
+      await nextTick();
 
-    expect(wrapper.emitted('filter-changed')[0][0]).toEqual({ severity: ids });
+      expect(wrapper.emitted('filter-changed')[0][0]).toEqual({ severity: ids });
+    });
   });
 });